Enzymes are remarkably selective catalysts. They bind a particular substrate out of many available compounds in solution, then they perform a reaction at a particular position of the bound substrate (thus showing regioselectivity), often with stereoselectivity as well. The geometric control in the enzyme-substrate complex can completely dominate the normal reactivity of the substrate. For example, enzymes in the class cytochrome P450 can hydroxylate unactivated carbons in steroids while leaving much more reactive substrate positions, such as those in or next to double bonds, untouched. (Soggon, 1996; Groves & Han, 1995; Meunier, 1992--). In these enzymes an oxygen atom becomes attached to the iron atom in the metalloporphyrin, and is then transferred to the substrate within the enzyme-substrate complex.
In addition to the interest in achieving a mimic of the great rate accelerations achieved in enzymatic catalysis, imitating the selectivity is at least as important. It is critical to understand the geometric control typical of enzyme reactions in selectively functionalizing steroids and other substrates. In the earliest work, for example, a benzophenone attached to a steroid was shown to perform selective photochemical functionalization of the substrate (Breslow and Winnik, 1969; Breslow and Baldwin, 1970; Breslow and Scholl, 1971; Breslow and Kalicky, 1971; Breslow, et al, 1973; Breslow, et al, 1978; Czarniecki and Breslow, 1979; Breslow, et al., 1981). In later work, a template attached to the substrate was able to direct free radical reactions to specific carbons because of the geometry of the template-substrate species; (Breslow, 1991; Breslow, 1988; Breslow, 1980). However, there were limitations to these methods.
For one, the reagent or template was covalently attached to the substrate, so catalytic turnover was not possible. As a corollary of this, relatively simple reagents or templates were used--attached by a single flexible link--so the geometric control was limited.
The ability of oxidizing enzymes, particularly those of the cytochrome P-450 class, to perform selective hydroxylations of unactivated carbons in substrates such as steroids is of great practical importance. It also represents a great challenge for biomimetic chemistry. Indeed the phrase biomimetic chemistry was first coined in 1972 with respect to efforts to achieve selective functionalization of steroids and other hydrocarbon derivatives with use of geometric control to mimic that in enzymes.(Breslow, 1972). However, early work involved the functionalization of steroids by reagents or catalysts that were covalently attached to a steroid hydroxyl group (Breslow, 1980). Thus catalytic turnover was not possible. Furthermore, the reactions were directed photolytic insertions, or directed free radical halogenations.
In nature, the relevant enzymatic reactions involve oxidation by metalloporphyrins, with reversible enzyme binding of the substrate in such a geometry that specific substrate positions are within reach of the oxygen atom on the metal. After oxidation, the product is released, so catalytic turnover is seen.
The present invention provides for the first time a true mimic of this entire process. The present invention describes that catalytic turnover with geometric control can be used to replace currently used methods of producing various pharmaceutically important compounds. For example, it is highly desirable to replace fermentation with a clean and straight-forward chemical process. In particular, pharmaceutically important corticosteroids can be produced by such a process in place of the currently used fermentation procedures.